This invention relates to an optical transmitter which is for use in an optical communication network and comprises a directly modulated semiconductor laser.
In optical communication, electric signals are ordinarily transmitted as an optical beam representative of mark and space codes. The semiconductor laser is supplied with a modulated injection current to produce an intensity modulated optical signal as the optical beam. For this purpose, the semiconductor laser is preliminarily supplied with a d.c. bias current of a level which is near to the oscillation threshold level of the semiconductor laser. An electric pulse current representative of the mark and the space codes is superimposed on the d.c. bias current to provide the modulated injection current. It is usual that the optical beam has a high intensity for each mark code and a low intensity for each space code. At any rate, a ratio of the high intensity to the low density is called an extinction ratio.
A paper was contributed by M. Shikata et al, including the present applicant, to a joint meeting of the 5th International Conference on Integrated Optics and Optical Fibre Communication (IOOC) and the 11th European Conference on Optical Communication (ECOC) held October 1985 in Venezia, Italy, and is included in Technical Digest, Volume III, pages 49 to 52, under the title of "1.5 .mu.m High Bit Rate Long Span Transmission Experiments Employing a High Power DFB-DC-PBH Laser Diode." In the manner described in the Shikata et al paper, the optical communication network has a degraded performance when the extinction ratio is poor. The d.c. bias current has therefore been selected a little lower than the oscillation threshold level particularly when the mark and the space codes are transmitted at a high bit rate, typically, at 1 Gb/s or higher.
Prior to the Shikata et al paper, a paper was contributed by J. E. Bowers et al to another joint meeting of Conference on Optical Fiber Communication (OFC) and Third International Conference on Optical Fiber Sensors (OFS) held February 1985 in San Diego, Calif., and is included as paper number MH3 in Technical Digest, pages 22 to 24, under the title of "Fiber-optic frequency discriminator for microwave optical communications." According to the Bowers et al paper, the semiconductor laser is used in producing a frequency modulated optical signal as the optical beam. In a counterpart optical receiver, an optical frequency discriminator is used in converting the frequency modulated optical signal to an intensity modulated optical signal. The optical frequency discriminator may typically be a Mach-Zehnder interferometer. This makes it possible to avoid degradation of the extinction ratio even when the d.c. bias current is selected above the oscillation threshold level. It is, however, mandatory to couple a polarization controller to the optical frequency discriminator. This is because the optical frequency discriminator has a polarization dependency.